In many applications a reflector having a particular cross-section is desired. Such cross-sections may be parabolic, spherical, ellipsoidal, or of other shapes depending upon the requirement of the application. Parabolic reflectors are particularly commonly required. That is because parabolic reflectors will provide a collimated beam of light from a point source.
In designing a light source utilizing a reflector, whether parabolic or of other shape, a focal length and aperture size must be selected. The choice of these two parameters then dictates the depth of the reflecting surface. A problem can arise when an application requires a reflector having a short focal length and a wide aperture. In order to obtain such a desired wide aperture with conventional reflectors, the reflector must be very deep, i.e., enclose a large volume. This can create severe problems when space for the reflector is limited. An example of a situation where such a problem arises is in the design of reflectors for use in automobile taillights.
One solution to this problem is to utilize a Fresnel-type reflector. A Fresnel-type reflector is typically a flat surface having structures in the form of straight or arcuate ridges and grooves which allow such a reflector to mimic the operation of a curved reflector. The problem with using a flat Fresnel-type reflector is that such reflectors are inefficient compared with true curved reflectors. This is because the curved reflector actually surrounds the light source and collects light which is emitted in many directions, while a flat reflector, although mimicking the optical properties of the curved reflector, is only able to colect light which is emitted in the direction of the plane of the reflector.
Another alternative which has been used is to provide a modified curved reflector. In such a reflector a first portion of the reflector will be curved to form a parabola having a short focal length. A second portion of the reflector will be curved to form a parabola of a longer focal length. The second portion includes a Fresnel structure which causes the second portion to mimic a parabolic reflector having the same focal length as the first portion of the reflector. This approach provides a reflector having a larger aperture than would be possible for the given focal length and depth of the reflector if a standard parabolic reflector were used. Reflectors of this type, however, still enclose an undesirably large volume.